Very cold this morning and car very reluctant to get up to normal temp,so I am wondering if it's worth making some sort of radiator cover to help keep some heat in and get up to a good operating temp.

I would only use it until engine is up to temp.

Any thoughts???

No. If it seems to tkae a very long time to warm up, or cools back down when moving, get the thermostat chnaged.

The thermostat should keep the rad out of circuit until the engine warms up.